Increasing demand for bandwidth-intensive wireless communication of voice, video, and data is straining point to point and backhaul radio networks, and cellular base stations, particularly in densely populated metropolitan areas. To reduce this strain, mobile operators are turning to the European Telecommunications Standard Institute (ETSI) “E-band” frequency spectrum to expand wireless network capacity at fiber-caliber data rates. The E-band frequency spectrum for point-to-point (PTP) (also known as “wireless backhaul”) includes two 5 GHz wide blocks: 71-76 GHz and 81-86 GHz. The ETSI E-band specification specifies a receiver (also referred to as a “down-converter”) be able to handle a “strong interferer” (i.e., 30 dB above threshold) anywhere in the two 5 GHz wide blocks (except for very close to the desired signal).
The strong interferer in turn creates a linearity specification called “IF/2” when a frequency of the strong interferer is half-way between a frequency of a local oscillator signal of a down-converting mixer of the E-band receiver and the desired radio frequency (RF) signal, hence at IF/2. For heterodyne receivers designed to operate over the 5 GHz blocks, the IF/2 linearity specification implies that the Intermediate Frequency (IF) be set greater than 10 GHz (i.e., 2×5 GHz=10 GHz). Because of the finite bandwidth of receiver filters, the IF value increases in a real system to set the IF to be more than 12 GHz, with 17 GHz being a possible choice.
One type of modern receiver has four IF outputs, representing 0, 90, 180, and 270 degrees, which are recombined. The four IF outputs are often called I, Q, I_bar, and Q_bar. In lower frequency systems (i.e., 18 GHz or 38 GHz ETSI point-to-point radios), the four IF outputs are at a much lower frequency {e.g., 1 to 3 GHz) and recombining the IF outputs is straightforward. In an E-band receiver with IF=17 GHz, recombining the IF outputs is difficult and costly.
It would be desirable to implement double down-conversion with multiple independent intermediate frequencies for E-band applications.